1. Field of the Invention
The present invention relates to disk drives. More particularly, the present invention relates to a disk drive employing asymmetric acceleration/deceleration pulses for acoustic noise reduction during unlatch.
2. Description of the Prior Art
FIG. 1A shows a prior art disk drive 2 comprising a disk 4 and a head 6 actuated over the disk 4 by a voice coil motor (VCM). The head 6 is connected to the distal end of an actuator arm 8 which is rotated about a pivot 10 in order to actuate the head 6 radially over the disk 4. The VCM comprises a voice coil attached to the actuator arm 8 having a first leg 12A and a second leg 12B for conducting current in a clockwise or counterclockwise direction thereby generating a magnetic flux which interacts with the magnetic field of permanent magnets (not shown) to generate a torque to rotate the actuator arm 8 about the pivot 10 in a clockwise or counterclockwise direction. The disk 4 comprises a plurality of tracks 14 defined by a plurality of embedded servo sectors 16, wherein disk drive control circuitry 17 processes the embedded servo sectors 16 in a closed-loop servo system to seek the head 6 to a target track and maintain the head 6 over the target track during read/write operations.
When the disk drive is powered down (or otherwise idle), the disk drive control circuitry 17 will perform a park operation wherein the head 6 is parked and the actuator arm 8 is latched. In the embodiment of FIG. 1A, the head 6 is parked on a landing zone 18 by rotating the actuator arm 8 in the counterclockwise direction. While the head 6 is parked, the actuator arm 8 is “latched” to prevent it from rotating the head 6 away from the landing zone 18. In the embodiment of FIG. 1A, a magnet 20 attached to a crash stop 22 attracts and holds a metal tang 24 attached to the actuator arm 8.
FIG. 1B shows a flow diagram of the steps executed by the disk drive control circuitry 17 to unlatch the actuator arm 8 when the disk drive is powered up (or otherwise comes out of an idle state). The actuator arm 8 is typically unlatched by driving the VCM with an open loop current since position (or velocity) information is unavailable. FIG. 1C shows a waveform illustrating the open loop current applied to the VCM to unlatch the actuator arm 8. Referring to FIG. 1B, when an unlatch operation is initiated at step 26, an acceleration pulse is applied to the VCM at step 28 to unlatch the tang 24 from the magnet 20. The acceleration pulse comprises an acceleration magnitude +M applied to the VCM and an acceleration interval A (FIG. 1C). At step 30 there is a predetermined delay for an interval B to allow the tang 24 to “escape” the latching force of the magnet 20. At step 32 a deceleration pulse is applied to the VCM to decelerate the actuator arm 8 to enable at step 34 closed loop position control of the actuator arm 8 by reading the embedded servo sectors 16. The deceleration pulse comprises a deceleration magnitude −M applied to the VCM and a deceleration interval C (FIG. 1C). The deceleration interval C is less than the acceleration interval A due to the force needed to escape the latching force of the magnet 20. The deceleration magnitude −M is equal (or nearly equal) the acceleration magnitude +M resulting in a “bang-bang” open-loop current profile.
Using a “bang-bang” open-loop current profile for unlatching the actuator arm can generate undesirable acoustic noise for certain applications, such as digital video recorders. There is, therefore, a need to attenuate the acoustic noise when unlatching the actuator arm in a disk drive using an open-loop current profile.